1. Field of the Invention
The present invention relates to a noncontact electric power transmission system, and more specifically to a design technique of, in a noncontact electric power transmission system comprising a power transmitter circuit section and a power receiver circuit section, clarifying a relationship between respective ones of a switching oscillating frequency in the power transmitter circuit section, an LC resonant frequency in the power transmitter circuit section, and an LC resonant frequency in the power receiver circuit section, to facilitate obtaining required characteristics.
2. Description of the Related Art
Heretofore, a resonant full-bridge system has been generally used in a type of noncontact electric power transmission system designed to achieve high efficiency in a contactless manner under a low DC input power.
In the resonant full-bridge system, a switching circuit is driven by an oscillating frequency of a control IC or the like, i.e., a switching frequency of the switching circuit is determined by the oscillating frequency. In terms of design parameters, there are the oscillating frequency of the control IC provided in a power transmitter circuit section of the system (power transmitter-side oscillating frequency), a resonant frequency of an LC series resonant circuit or an LC parallel resonant circuit provided in the power transmitter circuit section (power transmitter-side resonant frequency), and a resonant frequency of an LC parallel resonant circuit provided in a power receiver circuit section of the system (power receiver-side resonant frequency). JP 2001-103685A and JP 08-502640A disclose that a maximum electric power is transmitted at a resonance point when the power transmitter-side resonant frequency is set to become equal to the power receiver-side resonant frequency. As seen in these documents, it has been considered that optimum characteristics can be obtained when the three frequencies (the oscillating frequency, the power transmitter-side resonant frequency and the power receiver-side resonant frequency) have approximately the same values.
An output characteristic curve (a) illustrated in FIG. 1 represents an output voltage-output current characteristic to be obtained when the three frequencies (the oscillating frequency, the power transmitter-side resonant frequency and the power receiver-side resonant frequency) are set to become equal to each other. An output characteristic curve (b) illustrated in FIG. 1 represents an output voltage-output current characteristic of a noncontact electric power transmission system where a resonant capacitor is eliminated from each of the LC series or parallel resonant circuit in the power transmitter circuit section, and the LC parallel resonant circuit in the power receiver circuit section.
As seen in FIG. 1, if the oscillating frequency, the power transmitter-side resonant frequency and the power receiver-side resonant frequency are set to become equal to each other, an output voltage is apt to become higher at a resonance point A, even at the same current. This characteristic cannot be used without modification, because it is unfit for an output necessary as a commonly-used power supply. Moreover, if one of the three frequencies deviates from the resonance point, the output voltage will sharply drop and become unstable, which makes it impossible to obtain a stable output characteristic. Further, in cases where it is attempted to obtain a required output characteristic in an existing system, it is necessary to selectively set a value of the power transmitter-side oscillating frequency, a parameter value determining the power transmitter-side resonant frequency (i.e., an inductance value of a transmitter coil and a capacitance value of a capacitor in the power transmitter circuit section), and a parameter value determining the power receiver-side resonant frequency (i.e., an inductance value of a receiver coil and a capacitance value of a capacitor in the power receiver circuit section), on a case-by-case basis. In other words, it is necessary to adjust the power transmitter-side resonant frequency and the power receiver-side resonant frequency individually. If the two frequencies are adjusted in an imbalanced manner, transmission efficiency is likely to deteriorate. Furthermore, a design process becomes complicated and requires a lot of time.